Currently, a polyolefin-coated support sheet produced by coating both surfaces of a substrate consisting of a paper sheet with a polyolefin resin is widely employed as a support sheet for photographic printing paper. Since the polyolefin coating layer is hydrophobic, the above-mentioned type of support sheet is advantageous in comparison with baryta paper in that in develop-fixing treatment, the treating liquid is not allowed to easily penetrate into the support sheet, and thus the necessary time for water-rinsing and drying is significantly shortened. Also, the support sheet has an advantage that since the treating liquid does not penetrate into the substrate paper sheet, the shrinkage and elongation of the support sheet are restricted and thus the support sheet exhibits an excellent dimensional stability.
For the purpose of enhancing an opacifying power or image-forming property, the polyolefin resin in the coating layer of the support sheet is mixed with an inorganic white pigment such as titanium dioxide. The pigment has a poor dispersing property for the resin and causes a problem such that when the resin is melt-extruded, the resin is foamed by a volatile component contained in the pigment and thus the resultant resin coating layer is cracked or broken. For this reason, it is impossible to increase the content of the pigment in the coating layer to a high level sufficient to obtain a satisfactory opacifying power and the image-forming property.
Generally speaking, when titanium dioxide pigment is employed, it is difficult to add this pigment at a content of about 20% by weight or more. Therefore, the photographic printing paper produced from this type of support sheet for photographic printing sheet is unsatisfactory in sharpness of images.
Recently, a support sheet for photographic printing paper, having an electron beam-cured resin coated layer formed by coating a resin composition curable by an electron beam irradiation, namely an electron beam-curable resin, on a support sheet, was provided by, for example, Japanese Examined Patent Publication (Kokoku) No. 60-17,104, Japanese Examined Patent Publication (Kokoku) No. 60-17,105, and Japanese Unexamined Patent Publication (Kokai) No. 57-49,496. In accordance with this method, when the coating layer is formed, it is unnecessary to heat and melt the resin composition at a high temperature, and thus the pigment content can increase to 20 to 80% by weight. Therefore, the photographic printing paper sheet produced from the above-mentioned type of support sheet has a significantly enhanced image sharpness in comparison with the conventional photographic printing paper sheet having the polyolefin resin coating layers. Nevertheless, with respect to the photographic printing paper produced by coating a photographic photosensitive layer on an electron beam-cured resin-coating layer cured by an electron beam irradiation, it is known that in the developing treatment, a phenomenon that a photographic developing reagent is absorbed by and remains in the resin coating layer, and thus the photographic printing sheet is discolored yellow after the developing treatment, namely a yellowing phenomenon occurs, and when a developing treatment is applied to the photographic printing paper after a lapse of a storage period, the degree of fogging increases to a level not negligible for practical use or the photosensitivity thereof changes. On the other hand, it is known that the resultant coating layer is hard and has little flexibility and thus is sometimes cracked or broken.
There have been made various attempts to overcome the above-mentioned fogging problem. For example, Japanese Examined Patent Publication (Kokoku) No. 1-21,495 discloses a method of providing a polyethylene coating layer on an electron beam-cured resin coating layer to inhibit the change in the photosensitivity during storage. In this method, however, there is a problem that the fogging-reducing effect cannot be enhanced unless the thickness of the polyethylene coating layer is increased, and thus the enhancement in the image sharpness which is the largest benefit of the utilization of the electron beam-curing technology must be sacrificed.
Also, Japanese Unexamined Patent Publication (Kokai) No. 60-144,736 discloses a method of inhibiting the change in the photographic sensitivity by arranging an interception layer between a substrate and an electron beam-cured resin coating layer. This method is, however, unsatisfactory in fogging prevention when stored for a long period of time, if the material as disclosed in the publication is used for forming the interception layer.
To prevent the fogging and yellowing and to make the coating layer soft, various methods, in which specific electron beam-curable polymers or electron beam-curable monomers are employed, are disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 59-124,336 (acrylic acid ester monomer), Japanese Unexamined Patent Publication (Kokai) No. 60-70,446 (urethane resin having double bond), Japanese Unexamined Patent Publication (Kokai) No. 61-204,241 (diacrylate, triacrylate, and epoxidized acrylate), Japanese Unexamined Patent Publication (Kokai) No. 61-236,547 (tetraacrylic ester), Japanese Unexamined Patent Publication (Kokai) No. 62-61,049 (hexaacrylate ester), Japanese Unexamined Patent Publication (Kokai) No. 62-109,046 (polybutadiene-containing resin), Japanese Unexamined Patent Publication (Kokai) No. 62-141,543 (heterocyclic ring-containing acrylate) and Japanese Unexamined Patent Publication (Kokai) No. 2-47 (acrylate ester of addition product of polyhydric alcohol with acrylic acid polymer). However, even if these methods are used, the above-mentioned problems are not sufficiently solved.
With respect to the yellowing of the coating layer by the developing liquid, which has a close relationship to the fogging concentration and the flexibility of the coating layer, there is an opposite tendency in the yellowing due to exposure dose. Namely, when the electron beam is irradiated at a high exposure dose, the degree of yellowing of the coating layer by the developing liquid is restricted to a low level, the degree of fogging becomes higher, and thus the flexibility of the coating layer is reduced. On the other hand, at a low exposure dose, the fogging is inhibited and the flexibility of the coating layer is secured to a certain extent, and the degree of yellowing significantly increases, and the physical properties, for example, bonding property and mechanical strength of the coating layer are deteriorated.
Accordingly, to prevent the yellowing without reducing the physical properties of the coating layer, it is necessary to provide a structure of a new support sheet in which an appropriately selected electron beam-curable unsaturated organic compound is used, and even when the electron beam is applied in an exposure dose large enough to bring about the formation of crosslinkages in the coating layer, the fogging does not occur, and the flexibility of the coating layer and the mechanical properties and quality of the substrate paper sheet are not deteriorated, and a process for producing the same. This is very important to effectively eliminate all the above-mentioned problems simultaneously.